The composition and structure of a friction material consisting of a mat of fibres impregnated with a resin of a thermosetting type are described in detail in International patent specification No. WO95/26473. In particular, that document mentions the structural advantages and working advantages of such a type of material as compared with the materials in the state of the art, and particularly with respect to known materials of the "paper" type, which consist essentially of cellulose fibres and which are made using a humid process, by a common method used in paper manufacture, that is to say by processing of a dispersion of short fibres, in particular cellulose fibres, in an aqueous solution containing fillers.
Reference may also be made to the de-ailed contents of the above mentioned International patent specification in the context of working with such a material in order to make a friction liner in accordance with the invention.
This type of unwoven material, which is applicable to a dry process rather than the humid process employed to obtain a paper type material, has the structural advantage that it is porous and has porosities of high magnitude in the range between 50 and 500 micrometers, and that the cells or pores communicate with each other. By contrast, porosities of papers are much smaller, being generally between 5 and 50 micrometers, and their pores are not in communication with each other.
The concept of intercommunicating cells in the porous material is relative, that is to say it has to be considered with reference to the liquid in which the friction liner is immersed, that is to say, in most applications, the oil in a vehicle transmission. In this connection, the oil does not pass through a paper type material, while it does pass through the porous friction material made in accordance with International patent specification No. WO95/26473.
The porosity of this type of material enables oil to circulate within the actual interior of the friction liner, and in particular from its inner periphery to its outer periphery and vice versa, which enables the heat which is produced during operation, by friction of the liner, to be evacuated very effectively. As a result, it also prevents undue heating which could cause degradation of the friction material, or of the oil or the system in general in which the clutch having the liner is incorporated.
Such a beneficial feature is of particular relevance in systems with controlled or continuous sliding.
By contrast, this internal circulation of the oil within the liner may be detrimental in systems such as devices of the so-called "lock-up" type, in which the friction liner must also act as a seal in order to prevent the occurrence of counter-pressures in the lock-up chamber, which would lead to a reduction in the torque transmission capacity of the device concerned.